Coupled 3D Finite Element Modeling of Electromagnetic Free Expansion of Al Tube

Abstract

In automotive industry there is a general trend of decreasing the vehicle weight which encourages the use light weight metals like aluminium. As EMF can enhance the formability of aluminium it is becoming popular in automotive industry. The limitations of conventional forming like low formability increased wrinkling and springback can be overcome by EMF. As the driving force in EMF is the pulsed magnetic field, it is a contactless forming process which is another advantage over conventional forming. As the EMF process takes place in few microseconds it’s too hard to study it experimentally. EMF is simulated by EM module in LS-DYNATM to predict deformation, current wave pattern, and electromagnetic field. Electro-magnetic force is the driving force in electromagnetic forming process. It has been used in the forming of light weight and difficult to form metals like aluminium and magnesium alloy etc. As the process involves high strain rate, it has all the advantages of high-velocity forming process like increased formability, reduced springback and reduced wrinkling [1, 2]. In this technique large amplitude electric current (100 kA to 200 kA) is passed through the coil for few microseconds. The strain rate achieved is of the order of 10-3. Inertia force plays an important role in EMF reducing the wrinkling of the parts [3]. Electromagnetic forming involves thermal, mechanical and electromagnetic phenomenon. Conraux et al. [4] developed a formulation for a 3D magneto dynamic problem and presented constitutive equations that govern electromagnetism during electromagnetic forming. LSTC has developed EM module to numerically simulate electromagnetic forming [5]. Electromagnetic fields are solved by finite element method and surrounding air/insulators are taken care by boundary element method. M.A.Bahmani et al. [6] have carried out 3D simulations by FEA software MAXWELL and they are used to calculate the magnetic force distribution applied on the workpiece during the electromagnetic forming. G.Bartels et al. [7] has presented simulation approaches for the preliminary investigation of the electromagnetic metal forming process. He has compared an uncoupled simulation model to a more rigorous sequentialcoupled approach. According to G.Bartels et al. the simple loose coupled approach can only be used for relatively fast deformation process. Otherwise the more accurate sequential-coupled model should be used. Jianhui Shang et al. [8] have assessed the predictive ability of EM module in LS DYNA through comparison between experimental and numerical results of electromagnetic tube expansion. Electromagnetic forming process is carried out in few microseconds and it is difficult to find out the strain behaviour, velocity of workpiece, and deformation pattern experimentally. Current work is focusing on estimating few of these parameters numerically. The simulation of electromagnetic free expansion of Al tube is carried out with the help of EM module available in LS-DYNATM. Physics of EMF Maxwell’s Equations, form the basis of numerical computation in LS-DYNA software. The Maxwell’s Equations are given as follows